1. Field of the Invention
The present invention relates to devices and methods for identifying state of polarization of an electromagnetic wave and, more particularly, to applications where an identification of a state of polarization of an electromagnetic wave are employed, such as for target discrimination.
2. Related Art
Identifying the state of polarization of an electromagnetic wave by determining the Stokes polarization vector components of the wave is known. In particular, an electromagnetic wave, such as a spectral band of light, may be characterized as having four Stokes vector components (s0,s1, s2, and s3). The component s0 is proportional to the intensity of the wave. The components s1, s2, and s3 may be related to the orientation of the polarization, e.g., an ellipse and its ellipticity. In general, the orientation of polarization of a plane (planar phase front) monochromatic (single frequency) wave is elliptical. However, the ellipse may degenerate into a straight line in the case of linear polarization, and for circular polarization, the ellipse may degenerate into a circle.
An elliptically-polarized wave can be considered as the superposition of two waves of arbitrary orthogonal (perpendicular) polarization and amplitude a1 and a2 with phase difference xcex4. In this case, the components of a Stokes polarization vector (s0, s1, s2, and s3) may be equated to amplitude (a1 and a2) and phase difference (xcex4) as provided in the Table below:
Accordingly, based on the equations given above, the Stokes vector is known if the parameters a1, a2, and xcex4 are known. For further details concerning the Stokes polarization vector the reader is referred to the Principles of Optics, 3rd Edition, by M. Born and E. Wolf, Pergamon Press, Oxford, 1965, Chapter 1 which is incorporated herein by reference to the extent necessary to make and practice the present invention.
One way of measuring the Stokes vector components (s0, s1, s2, and s3) is to place two polarizers and a retarder in the optical path sequentially. Insertion of a first polarizer into an optical path gives a measure of one of the linear polarizations and a second polarizer is also inserted to give the other linear polarization. A retarder is further inserted into the optical path to retard a signal having a given sense of polarization in phase relative to a signal having another sense, where the two senses are generally orthogonal to each other. Output from the retarder is a signal containing data that can be used to calculate xcex4 when the linear components are known. The disadvantage of this approach is that it involves moving parts, since these optical components must be placed successively in the optical path. Also, in a dynamic scene, a polarimeter using moving parts would give smeared results, since the scene could change during the times that the polarizers are being changed.
Other ways of measuring the Stokes vector components have been proposed. For example, the paper entitled xe2x80x9cSpectroscopic Polarimetry with a Channeled Spectrumxe2x80x9d by Kazuhiko Oka and Takayuki Kato, published in Optics Letters, Vol. 24, No. 21, Nov. 1, 1999 describes a system for spectropolarimetry which eliminates the need for inserting and removing polarizers into or out of the optical path. In particular, Oka and Kato employ a pair of birefringent retarders and an analyzer to modulate light so that the state of polarization of the light varies with frequency. The modulated light is then passed to a spectrometer or spectrum analyzer and then to a computer where, through Fourier analysis, the state of polarization of the modulated light is determined.
It is also known that light reflected or emitted from a man-made object will generally have a different polarization signature from light reflected or emitted from a natural background. For example, unpolarized light incident on a flat surface will have much of its vertical component absorbed and its horizontal component reflected. However, to date, no suitable device or method of measuring a complete polarization signature of a target scene with no moving parts is available.
In accordance with an embodiment of the present invention, a polarimeter is provided for measuring the polarization state of a wideband electromagnetic signal. The polarimeter includes a first antenna for receiving the electromagnetic signal and a modulator. The modulator is interconnected with the first antenna for modulating the electromagnetic signal whereby a modulated electromagnetic signal results which contains a different polarization state for each frequency of the electromagnetic signal and wherein the amplitude of each frequency component of the modulated electromagnetic signal is a function of the particular polarization state of each frequency component of the electromagnetic signal. Also, the modulator may be configured to modulate at a radar frequency. A linear polarizer is also provided which may be configured to pass a first predetermined polarization of the modulated electromagnetic signal through a first output thereof. Further, a first receiver is provided which includes a detector for receiving and demodulating the modulated electromagnetic signal from the linear polarizer.
In accordance with another aspect of the present invention, a target discriminator for measuring the polarimetry of a wideband electromagnetic signal reflected from a target is provided. The target discriminator may comprise a first antenna for receiving the electromagnetic signal and a modulator interconnected with the first antenna for modulating the electromagnetic signal. The modulator may comprise a first optically thick retarder of birefringent material wherein a fast and a slow axes of the first retarder define respective x and y axes of a rectangular coordinate system. The modulator may also comprise a second optically thick retarder of birefringent material and wherein the fast axis of the first optically thick retarder forms an angle of approximately forty-five degrees to the fast axis of the second optically thick retarder. A linear polarizer may be configured to pass a first predetermined polarization of the modulated electromagnetic signal through a first output thereof and to pass a second predetermined polarization of the modulated electromagnetic signal through a second output thereof. A first receiver may be provided which comprises a detector for receiving and demodulating the modulated electromagnetic signal from the linear polarizer. An analog to digital converter may be connected to the detector for converting the modulated electromagnetic signal into a digital word. A processor may be further provided and the processor may be configured to receive the digital word and calculate at least one component of a Stokes polarization vector of the electromagnetic signal and wherein the processor further calculates a degree of depolarization of the electromagnetic signal identifiable as a target signature. The target discriminator may further comprise a transmitter for transmitting the electromagnetic signal, a second receiver employed to receive the second predetermined polarization of the modulated electromagnetic signal and a power combiner that is employed to combine the modulated electromagnetic signal received by each of the first and second receivers.
In accordance with a further aspect of the present invention, a polarimeter for measuring the polarization state of a wideband electromagnetic signal is provided, the polarimeter may comprise means for receiving the electromagnetic signal; means for modulating the electromagnetic signal being interconnected with the receiving means whereby a modulated electromagnetic signal results which contains a different polarization state for each frequency of the electromagnetic signal with the amplitude of each frequency component of the modulated electromagnetic signal being a function of the particular polarization state of each frequency component of the electromagnetic signal, the modulator being configured to modulate at a microwave frequency; means for linearly polarizing the modulated electromagnetic signal; and means for demodulating the modulated electromagnetic signal output from the linear polarizer.
In yet a further aspect of the present invention, a method for measuring the polarization state of a wideband electromagnetic signal, the method comprises receiving the electromagnetic signal; modulating the electromagnetic signal to obtain a modulated electromagnetic signal containing a different polarization state for each frequency of the electromagnetic signal with the amplitude of each frequency component of the modulated electromagnetic signal being a function of the particular polarization state of each frequency component of the electromagnetic signal, the electromagnetic signal being modulated at a microwave frequency; linearly polarizing the modulated electromagnetic signal; and demodulating the modulated electromagnetic signal output from the linear polarizer.